Hydrogen is the cleanest and most efficient fuel used in fuel cells. It is widely used in low temperature fuel cells like proton exchange membrane (PEM) fuel cells, alkaline fuel cells and phosphoric acid fuel cells, because its oxidation rate at the anode is high, even at room temperature. However, producing pure hydrogen is not a trivial task. Hydrogen is normally produced through reforming hydrocarbon fuels, such as methane, propane, and methanol. This not only makes the entire fuel cell system more complicated, it also dramatically increases the cost. Moreover, any carbon monoxide (CO) remaining in the reformed gas, even at ppm levels, will poison the electrodes of a PEM fuel cell and reduce its performance. In addition, transporting and storing hydrogen is very difficult, presenting a safety hazard.
The problems associated with hydrogen have encouraged scientists to look for other fuels that can be directly oxidized without requiring a reforming step. Methanol, the simplest alcohol containing only one carbon atom, is the most popular and widely used fuel in this regard. A direct oxidation fuel cell using methanol as the fuel is called a direct methanol fuel cell (DMFC).
In U.S. Pat. Nos. 3,013,908 and 3,113,049, a DMFC is described. Liquid feed direct methanol fuel cells have been in use from the early 1960s. These early DMFCs used liquid electrolyte like a dilute sulfuric acid for proton transportation. Major problems were encountered using the sulfuric acid electrolyte, such as corrosion of cell materials, poisoning of the electrodes by the adsorption of sulfate anions, and leakage of the electrolyte through the surrounding materials. For example, the electrolyte could gradually leak out through the pores of the air cathode, also causing fuel loss and cathode poisoning.
In order to alleviate leakage, a solid proton exchange membrane was interposed between the anode and cathode. Nafion® perfluorinated polymer, made by E. I. DuPont, was used in U.S. Pat. Nos. 4,262,063 and 4,390,603.
U.S. Pat. No. 4,478,917 used a membrane comprising styrene-divinylbenzene co-polymers with sulphonate groups.
In recent years, the use of liquid electrolyte has not been frequent in a DMFC. U.S. Pat. No. 5,599,638 was granted to Surampudi et al for just using a proton exchange membrane like Nafion as the electrolyte. Nafion membranes have excellent chemical, mechanical, thermal, and electrochemical stability and their ionic conductivity can reach as high as 0.1 S/cm. The kinetics of methanol oxidation and oxygen reduction at the electrode/membrane/electrode interfaces has been found to be more facile than at the electrode/sulfuric acid/electrode interfaces. Corrosion of cell materials becomes less severe since the fuel and water solution is free from sulfuric acid. The Nafion cell can be operated at temperatures as high as 120° C., while a sulfuric acid cell tends to degrade at temperatures higher than 80° C. Also, the absence of conducting ions in the fuel and water solution substantially eliminates the parasitic shunt currents in a multi-cell stack. Such a fuel cell is illustrated in U.S. Pat. No. 6,248,460, a continuation of U.S. Pat. No. 5,599,638, granted to Surampudi et al.
In U.S. Pat. No. 5,904,740, granted to Davis, a fuel cell with formic acid added into the methanol and water solution for the conduction of protons within the anode structure is shown. The formic acid is claimed to improve ionic conductivity and to be a clean burning fuel that does not poison the catalysts.
Unfortunately, methanol poses the serious problem of penetrating and crossing through Nafion membranes as well as other types of proton exchange membranes, via physical diffusion and electro-osmotic proton drag. Such crossover not only results in a large waste of fuel, it also greatly lowers cathode performance. Most of the methanol crossover will be electrochemically and chemically oxidized at the cathode. These oxidation reactions not only lower the cathode potential, they also consume some oxygen. Should the reaction intermediate comprise carbon monoxide, it can be adsorbed onto the catalyst surface, thus poisoning the cathode. This will further lower the performance of the fuel cell.
In U.S. Pat. No. 5,672,438, a thin layer of polymer having a higher ratio of backbone carbon atoms to those of the cationic exchange side chain is illustrated. This polymer reduces the methanol crossover rate, although the membrane resistance increases. It was suggested that the polymer with higher carbon atom ratios be preferably orientated on the anode side. Prakash et al described a polymer membrane composed of polystyrene sulfonic acid (PSSA) and poly(vinylidene fluoride) (PVDF), in WO 98/22989. The PSSA-PVDF membrane exhibited lower methanol crossover, translating into higher fuel and fuel cell efficiencies. Pickup et al suggested a modified ion exchange membrane possessing lower methanol crossover, in WO 01/93361A2. Existing membranes comprising Nafion were modified in situ by polymerizing monomers, such as aryls, heteroaryls, substituted aryls, substituted heteroaryls, or combinations thereof. The modified membrane exhibited reduced permeability to methanol crossover, often without a significant increase in ionic resistance.
Another barrier to the commercialization of DMFCs is the sluggishness of the methanol oxidation reaction. Moreover, some intermediates from methanol oxidation, like carbon monoxide, can strongly adsorb to the surface of catalysts, poisoning them, as aforementioned. Platinum alloys such as Pt/Ru have a much higher CO-tolerance, so they are widely used as the anode catalyst. Other short chain organic chemicals like formic acid, formaldehyde, ethanol, 1-propanol, 1-butanol, dimethoxymethane, trimethoxymethane, and trioxane have been suggested as fuels in direct oxidation fuel cells. U.S. Pat. No. 5,599,638 describes experimental results of using dimethoxymethane, trimethoxymethane, and trioxane in fuel cells. It was claimed that dimethoxymethane, trimethoxymethane, and trioxane could be oxidized at lower potentials than methanol, and thus would be better fuels than methanol. It was also claimed that only methanol was found to be the intermediate product from the oxidation of these fuels, thus there was no concern (i.e., methanol is ultimately oxidized to carbon dioxide and water). Using Nafion 117® as the membrane and oxygen as the oxidant, with a pressure of 20 psig, cell voltages of 0.25 V, 0.50 V, and 0.33 V were achieved at a current density of 50 mA/cm2, when dimethoxymethane, trimethoxymethane, and trioxane were used at cell temperatures of 37° C., 65° C., and 60° C., respectively. However, these performances are very low and would not provide a commercial fuel cell.
The present invention provides a direct oxidation fuel cell performing much better than DMFCs, using secondary alcohol as the fuel.
It is an object of this invention to provide a fuel cell using secondary alcohols as the fuel.
It is another object of the invention to provide a fuel cell using 2-propanol as the fuel.
It is yet another object of this invention to provide a fuel cell whose fuel crossover is much less than a DMFC using methanol.